“Electronic components”, such as integrated circuits (ICs), discrete components, and passive components, can be physically and electrically coupled to a substrate such as a printed circuit board (PCB) to form an “electronic assembly”. The “electronic assembly” can be part of an “electronic system”. An “electronic system” is broadly defined herein as any product comprising an “electronic assembly”. Examples of electronic systems include computers (e.g., desktops, laptops, hand-helds, servers, Web appliances, routers, etc.), wireless communications devices (e.g., cellular phones, cordless phones, pagers, etc.), computer-related peripherals (e.g., printers, scanners, monitors, etc.), entertainment devices (e.g., televisions, radios, stereos, tape and compact disc players, video cassette recorders, camcorders, digital cameras, etc.), and the like.
In the field of electronic systems there is an incessant competitive pressure among manufacturers to drive the performance of their equipment up while driving down production costs. This is particularly true regarding the packaging of electronic components on substrates, where each new generation of board-level packaging must provide increased performance while generally being smaller or more compact in size.
A substrate typically includes a number of insulation and metal layers selectively patterned to provide metal interconnect lines (referred to herein as “traces”), and a plurality of electronic components mounted on one or more surfaces of the substrate and functionally interconnected through the traces. The routing traces typically carry signals that are transmitted among the electronic components of the system. Some ICs have a relatively large number of input/output (I/O) pads. The large number of I/O pads requires a relatively large number of routing traces. Some PCBs require multiple layers of routing traces to accommodate all of the system interconnections.
Routing traces located within different layers are typically connected electrically by vias formed in the substrate. A via can be made by first forming vertically-aligned via pads in some or all layers of a PCB as it is being built up, forming a hole through the via pads, and then coating or plating the interior hole surface with an electrically conductive material, such as copper, silver, or gold.
One of the conventional ways of mounting components on a substrate is called surface mount technology (SMT). SMT components have terminations or leads (generally referred to as “electrical contacts”, “bumps”, or “pads”) that are soldered directly to the surface of the substrate. SMT components are widely used because of their compact size and simplicity of mounting. One conventional type of SMT component utilizes a ball grid array (BGA) to connect to the substrate. A BGA component has on one of its surfaces a plurality of solder balls, one ball per pad.
The electrical contacts of an SMT component are coupled to corresponding electrically conductive mounting or bonding pads (also referred to herein as “lands”) on the surface of the substrate, in order to establish secure physical and electrical connections between the component and the substrate. Ordinarily one land is dedicated to one SMT electrical contact. However, if the land needs to be connected to a trace on a different layer, it is necessary to additionally provide a via pad that is located adjacent the land and connected to the land through a trace on the same layer. This in effect can increase the PCB real estate consumed by lands and associated via pads by up to a factor of 2.
In order to fabricate PCBs in which components are mounted in higher densities, it is known to use a via-in-pad (VIP) structure. In this structure, a via is formed substantially completely within each mounting pad or land, thus conserving valuable “real estate” on the PCB that would otherwise be separately occupied by the vias and the lands. The resulting electronic system can be manufactured at a lower cost and in a more compact size, and it is therefore more commercially attractive.
Before the SMT component is mounted on a substrate, the substrate lands are selectively coated with solder paste. To mount an SMT component to a substrate, the component is carefully positioned or “registered” over the substrate so that its electrical terminals or contacts are aligned with the corresponding lands. Finally, in an operation known as “solder reflow”, the component terminals and the PCB lands are heated to a temperature that melts the solder paste, so that the terminals and lands make proper electrical and physical connections.
As will be discussed in greater detail below, mounting electronic components on currently used VIP structures can result in significant manufacturing defects.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a significant need in the art for apparatus and methods for mounting components to a substrate that offer relatively high density and high quality interconnections at a reasonable production cost.